add include guard

hysop/operator/diffusion.py

@@ -13,7 +13,10 @@ from hysop.parameters.scalar_parameter import ScalarParameter
 from hysop.operator.base.spectral_operator import SpectralComputationalGraphNodeFrontend
 from hysop.backend.host.python.operator.diffusion   import PythonDiffusion
 from hysop.backend.device.opencl.operator.diffusion import OpenClDiffusion
-from hysop.backend.host.fortran.operator.diffusion  import DiffusionFFTW
+try:
+    from hysop.backend.host.fortran.operator.diffusion  import DiffusionFFTW
+except ImportError:
+    DiffusionFFTW = None
 
 class Diffusion(SpectralComputationalGraphNodeFrontend):
     """

hysop/operator/poisson.py

@@ -12,7 +12,10 @@ from hysop.topology.cartesian_descriptor import CartesianTopologyDescriptors
 from hysop.operator.base.spectral_operator import SpectralComputationalGraphNodeFrontend
 from hysop.backend.host.python.operator.poisson import PythonPoisson
 from hysop.backend.device.opencl.operator.poisson import OpenClPoisson
-from hysop.backend.host.fortran.operator.poisson  import PoissonFFTW
+try:
+    from hysop.backend.host.fortran.operator.poisson  import PoissonFFTW
+except ImportError:
+    PoissonFFTW = None
 
 class Poisson(SpectralComputationalGraphNodeFrontend):
     """

hysop/operator/poisson_curl.py

@@ -10,9 +10,12 @@ from hysop.fields.continuous_field import Field
 from hysop.topology.cartesian_descriptor import CartesianTopologyDescriptors
 
 from hysop.operator.base.spectral_operator import SpectralComputationalGraphNodeFrontend
-from hysop.backend.host.fortran.operator.poisson_curl  import FortranPoissonCurl
 from hysop.backend.host.python.operator.poisson_curl   import PythonPoissonCurl
 from hysop.backend.device.opencl.operator.poisson_curl import OpenClPoissonCurl
+try:
+    from hysop.backend.host.fortran.operator.poisson_curl  import FortranPoissonCurl
+except ImportError:
+    FortranPoissonCurl = None
 
 class PoissonCurl(SpectralComputationalGraphNodeFrontend):
     """